Process of curing polymerizable resins
having terminal vinyl ester groups
using high energy electrons

ABSTRACT

A PROCESS OF COATING A POROUS OR NONPOROUS SUBSTRATE WITH A LIQUID POLYMERIZABLE FILM AND SUBJECTING THE COATED SUBSTRATE TO IONIZING RADIATION SUFFICIENT TO IMPART TO THE COATING COMPOSITION A DOSE OF FROM 1 TO 10 MEGARANDS IN ONE SECOND OR LESS. THE COATING COMPOSITIONS, CAPABLE OF SUBSTANTIALLY COMPLETE POLYMERIZATION IN LESS THAN ONE SECOND, CONTAIN UNDILUTED VINYL ESTER RESINS HAVING TERMINAL VINYL ESTER GROUPS, OR THE ABOVE DISSOLVED IN VINYL MONOMERS. THE VINYL ESTER RESINS ARE MADE BY REACTING: (1) A POLYFUNCTIONAL MATERIAL SELECTED FROM THE GROUP CONSISTING OF (A) DICARBOXYLIC ACIDS OR ACID CHLORIDES HAVING FROM 4 TO 15 CARBON ATOMS, AND (B) (POLYEPOXIDES HAVING TERMINAL, REACTIVE GLYCIDYL GROUPS, (C)) POLYFUNCTIONAL ISOCYANATES HAVING TERMINAL, REACTIVE ISOCYANATE GROUP (, OR (D) DICARBOXYLIC ACID ESTERS OF POLYEPOXIDES, POLYAMINES, POLYISOCYANATES,) WITH 2-HYDROXYALKYL ACRYLATES OR METHACRYLATES, OR (2) A HALF ESTER OF 2-HYDROXYALKYL ACRYLATE(S) OR METHACRYLATE(S) AND A DIBASIC ACID WITH A POLYEPOXIDE. PARTICULARLY USEFUL ARE COATING COMPOSITIONS CONTAINING RESINS HAVING TERMINAL ACRYLATE GROUPS AND AN ACRYLATE MONOMER, THE POLYFUNCTIONAL ACRYLATE HAVING MORE THAN ONE 2-OXYALKYLACRYLATE END GROUP PER MOLECULE WITH THE END GROUPS BEING JOINED BY A HYDROPHOBIC MOLECULE SUCH AS A DICARBOXYLIC ACID, POLYFUNCTIONAL ISOCYANATE OR POLYEPOXIDE.

United States Patent PROCESS OF CURING POLYMERIZABLE RESINS HAVING TERMINAL VINYL ESTER GROUPS USING HIGH ENERGY ELECTRONS Lewis S. Miller, Bellevue, Wash., assignor to Weyerhaeuser Company, Tacoma, Wash.

No Drawing. Original No. 3,560,237, dated Feb. 2, 1971,

Ser. No. 721,152, Apr. 15, 1968. Applicah'on for reissue Oct. 28, 1971, Ser. No. 193,580

Int. Cl. B44d 1/50 US. Cl. 11793.31 16 Claims Matter enclosed in heavy brackets II] appears in the original patent but forms no part of this reissue specification; matter printed in italics indicates the additions made by reissue.

ABSTRACT OF THE DISCLOSURE A process of coating a porous or nonporous substrate with a liquid polymerizable film and subjecting the coated substrate to ionizing radiation sufiicient to impart to the coating composition a dose of from 1 to megarads in one second or less. The coating compositions, capable of substantially complete polymerization in less than one second, contain undiluted vinyl ester resins having terminal vinyl ester groups, or the above dissolved in vinyl monomers. The vinyl ester resins are made by reacting:

(l) A polyfunctional material selected from the group consisting of (a) dicarboxylic acids or acid chlorides having from 4 to carbon atoms, and (b) [polyepoxides having terminal, reactive glycidyl groups, (0)] polyfunctional isocyanates having terminal, reactive isocyanate group or (d) dicarboxylic acid esters of polyepoxides, polyamines, po1yisocyanates,] with Z-hydroxyalkyl acrylates or methacrylates; or

(2) a half ester of Z-hydroxyalkyl acrylateIIs] or methacrylateIIs] and a a'ibasz'c acid with a polyepoxide.

Particularly useful are coating compositions containing resins having terminal acrylate groups and an acrylate monomer, the polyfunctional acrylate having more than one 2-oxyalkylacrylate end group per molecule with the end groups being joined by a hydrophobic molecule such as a dicarboxylic acid, polyfunctional isocyanate or polyepoxide.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to a process of curing coating compositions having terminal unsaturation in a very short amount of time using high energy electrons.

(2) Description of the prior art It is known that many polymeric materials, when subjected to high energy radiation in the form of high energy electrons, can be rapidly cured. Electron irradiation has a number of distinct advantages over conventional methods of polymerization and cross-linking using catalytic agents, especially when used to cure coatings on wood and wood products, fabrics, rubber, glass, and other similar substrates. Use of electron irradiation to cure polymeric coatings avoids the sometimes lengthy induction period associated with catalyst-induced cure, enables the use of polymeric coating compositions having indefinite pot life, produces little temperature rise during polymerization, and produces, in many cases, a superior coating-to-substrate bond.

One of the problems associated with electron irradiation is in finding a polymeric material which is suitable for the purpose desired and curable to a hard, tack-free condition in a short amount of time. With particular reference to the coating of wood and wood products, it is desirable to have a coating composition which is flexible, water resistant, and impact resistant. In addition, it is necessary that the polymeric composition cure or polymerize in a short amount of time at reasonable dose rates in order to make the process economically feasible.

To obtain fast curing a cross-linking mechanism is needed. In the present coating composition this is furnished by acrylates. It is known that diacrylates made by reacting polyhydric alcohols, such as ethylene glycol, with acrylic or methacrylic acids are curable by high energy radiation, as disclosed in US. Pat. No. 2,921,006. The diacrylates disclosed, however, do not cure to a hard tack free state using economically feasible dose rates, are too brittle, and swell on exposure to moisture and weathering, making them unacceptable for coating wood and wood products. Other polyester coating materials curable by high energy radiation are disclosed in US. Pat. No. 3,247,012. These compositions also lack certain desired properties, making them unacceptable for coating of wood and wood products.

SUMMARY OF THE INVENTION This invention is directed to a process for coating a substrate with a composition containing a polymerizable material having terminal vinyl ester groups and thereafter subjecting the coating to high energy ionizing radiation in the form of electrons to cure the coating to a tack-free condition in times of one second or less. It pounds could be cured with high energy electrons at reasonable dose rates in such a short amount of time. Further, this invention is directed to a process of coating wood and wood products with vinyl ester resins which, on curing, have good flexibility, excellent water resistance, and excellent impact resistance. The vinyl ester resins of this invention can be used undiluted or dissolved in 0 to 50% by weight of a vinyl monomer. Pigments and other conventional additives can be added to the coating compositions. Vinyl ester resins made by reacting:

(1) A polyfunctional material selected from the group consisting of (a) dicarboxylic acids or acid chlorides having from 4 to 15 carbon atoms, and (b) [polyepoxides having terminal, reactive glycidyl groups, (0)] polyfunctional isocyanates having terminal, reactive isocyanate groups or (d) dicarboxylic acid esters of polyepoxides, polyamines, or polyisocyanates] with Z-hydroxyalkyl acrylates or merhacrylares; or

(2) a half ester of 2-hydroxyalkyl acrylatelIs] or methacrylateIIs] and a dibasic acid with a polyepoxide are suitable in the process of this invention.

Using compounds of this type cure rates of less than A second at a dose rate of 3 megarads have been obtained.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Polymerizable vinyl ester resins described previously are liquids which may be used undiluted or dissolved in vinyl monomers which compolymerize with the vinyl ester resin upon subjection to irradiation. These resinous compositions are made by reacting Z-hydroxy'alkyl acrylates with polyfunctional, hydrophobic materials. The polyfunctional hydrophobic [group] material may be (1) an aliphatic saturated dicarboxylic acid having from 4 to [14] 15 carbon atoms, such as propanedioic acid (malonic acid), butanedioic acid (succinic acid), pentanedioic acid (sebacic acid), hexanedioic acid (adipic acid), octanedioic acid (suberic acid); (2) aromatic dicarboxylic acids such as phthalic acid; (3) a conventional epoxide compound having a 1,2-epoxy equivalency greater than 1 and having more than one epoxy group per molecule, for example, bis phenol A diglycidyl ether or other epoxy compounds obtained by reacting dihydric phenols with polyfunctional etherifying agents such as epichlorohydrin (described more fully in US. Pats. 2,467,171; 2,538,072; 2,582,985; 2,615,007; and 2,698,314); (4) [an epoxy compound recated with a dibasic acid or anhydride, such as maleic acid or anhydride, salicylic acid or anhydride, fumaric acid, itaconic acid, phthalic acid or anhydride, an aliphatic, saturated] a dicarboxylic acid chloride having from 4 to 15 carbon atoms; (5) polyurethane prepolymers made by reacting an organic polyfunctional isocyanate, for example, toluene diisocyanate, with polyols such as propylene or polypropylene glycol or polyether polyols; and (-6) organic polyfunctional isocyanates including aromatic, aliphatic, aralkyl, alkaryl and cycloaliphatic polyfunctional isocyanates. T hese compositiorts are prepared by condensing 0.8 to 1.2 gram molecular propotrions of the polyfunctional material per 1.8 to 2.5 gram molecular proportions of the Z-hydroxyalkyl acrylate or methacrylate in the absence of a solvent. [More specifically] Further, vinyl ester resins disclosed in UJS. Pat. No. 3,367,992, which is hereby incorporated by reference, are applicable in the process of this invention. Such resins are prepared according to said patent by reacting a half ester of a Z-hydroxyalkyl acrylate or methacrylate and a dibasic acid with a polyepoxide.

Specific examples of these materials include adipoylbis (2-oxyethylacrylate); the half ester condensation product of 2-hydroxyethyl acrylate[,] and maleic anhydride and] reacted with a bis-phenol A diglycidyl ether; the condensation product of polypropylene glycol, toluene diisocyanate, and Z-hydroxyethyl acrylate; the half ester condensation product of Z-hydroxypropyl acrylate L] and maleic anhydride and] reacted with butanediol diglycidyl ether; the half ester condensation product of Z-hydroxyethyl acrylate and maleic anhydride [and] reacted with a brominated bisphenol A-based polyepoxide resin with an epoxide equivalent weight of 350-400; [the condensation product of Z-hydroxypropyl acrylate, maleic anhydride, a bisphenol A-based polyepoxide having an epoxide equivalent weight of l86-l92, and phthalie anhydride]; and the like. [These compounds are prepared by condensing 0.8 to 1.2 gram molecular proportions of the polyfunctional hydrophobic group per 1.8 to 2.5 gram molecular proportions of the Z-hydroxyalkyl acrylate or methacrylate in the absence of a solvent.]

The vinyl ester resin or resins may be spread on the substrate undiluted or dissolved in up to 50% by weight of a vinyl monomer. Preferably an acrylate monomer is used in combination with the vinyl ester compounds to achieve optimum results. Small amounts of styrene, vinyl toluene, t-butyl styrene, chlorostyrene, hydroxyalkyl methacrylates or alkyl methacrylates having 1 to 8 carbon atoms may be used in admixture with the acrylate monomer; however, the presence of too great a quantity of the above compounds lowers the cure rate significantly and affects the properties of the cured coating.

Acrylate monomers which may be used include alkyl acrylates having from 1 to 8 carbon atoms, hydroxyalkyl acrylates, cycloalkyl acrylates, or glycidyl acrylates, such as acrylic acid, Z-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, glycidyl acrylate, cyclohexylacrylate, diacetonacrylamide, and others. It is preferred to use an acrylate monomer, such as n-butyl acrylate, which is readily available, economical, and has a low volatility. The monomer, if used, should be added in an amount which does not thin the coating composition to an extent that it penetrates the substrate too rapidly, if a porous substrate is used. Also, addition of too much monomer impairs the properties of high impact and water resistance present furnished by reason of the polyfunetional acrylate or methacrylate composition. Preferably, amounts up to 40% by weight or less are used.

The coating compositions as defined can be used to coat any suitable porous or nonporous substrate such as wood or wood products, plastics, glass, rubber, metals, etc. The properties of the resins are best utilized, however, in the coating of wood and wood products.

No solvent is necessary for the coating compositions. Polymerization of the coating composition is initiated and completed by ionizing radiation in the form of high energy electrons. To cure the coating composition, it is necessary to impart to the coating an ionizing radiation dose rate of from 1 to 10 megarads in 1 second or less. Generally, the electron beam is maintained in the range of 250,000-550,000 volts with sufiicient milliamperes per inch width of scan to impart l to 10 megarads to the coating composition in 1 second or less. A number of commercially available high electron beam machines are available and can be used including those sold by General Electric Company, Radiation Dynamics (Dynacote), Texas Nuclear Company, and High Voltage Engineering (ICT 500). For example, using the High Voltage Engineering (ICT 500) machine operated at 300 kilovolts, 20 milliamperes and a beam scan of 18 inches width, the current density is 20/18 or 1.1 milliamperes per inch of width. As the substrate passes under the beam at a distance of, for example, 2 /2 inches below the window of the unit and at right angles to the scan, it is exposed to significant portions of the beam for a distance of about 4 inches. If the coated substrate is traveling at, for example, 60 feet per minute or 1 foot per second, it covers the 4 inches in /s second. The exposure times in the examples were computed in such a manner.

Films of varying thickness can be cured depending on the voltage used. At about 300 kv. films ranging up to 12 mils in thickness can be easily cured. At about 500 kv. films up to 35 mils in thickness can be cured.

It is preferable to maintain a distance between the coated substrate and the window of the machine through which the electron beam passes from about 6 inches to the minimum necessary for clearance of the coated substrate.

Using certain coatings, cure in the presence of oxygen may leave a soft surface film on the coating. This may be prevented by conventional means such as using an inert atmosphere or irradiating the coated substrate under a covering film of release paper or plastic.

The coating compositions of this invention may be added to the substrate by spraying dipping, brushing or any other suitable means. Clear coatings on hardwood surfaces having up to 35 mils in thickness have been made using curing times of 1 second and less with the resinous compositions of this invention. Such a fast cure rate is advantageous when the high energy radiation is used in a conventional coating line operating at speeds from 20 feet per minute to feet per minute.

Pigments, dyes, and other suitable materials may be added to the resinous coating compositions if desired.

The following examples are given by way of illustration and are not intending to be limiting in any manner.

EXAMPLE I A solution of 69.6 grams (0.6 mole) of 2-hydroxyethyl acrylate in 47.5 grams of pyridine and ml. of benzene was placed in a flask equipped with a stirrer, dropping funnel and condenser. A solution of 54.9 grams (0.3 mole) of adipoyl chloride in 75 ml. of benzene was added with stirring and cooling. After the addition was complete, the mixture was heated one-half hour on a steam bath. The clear solution was decanted from the precipitate and passed through a column of activated alumina. After removing the benzene on a steam bath, 82.8 grams of a clear residue, adipoyl-bis(2-oxyethylacrylate) remained. A sample of Wood and aluminum was given a 0.005 to 0.010 inch thick coating of the above resin composition and the coating covered with polyethylene terephthalate (Mylar) film 5 mils in thickness. The covered products where then passed under a two million volt electron beam from a Van deGralf accelerator in a way that the samples received 1.7 megarads in 0.3 seconds. After removing the Mylar cover, a clear hard coating remained on the substrates which did not appreciably soften in water and was not cracked or crazed by a sharp hammer blow.

A subsequent coating on aluminum of the same material was irradiated with a 500,000 volt electron beam giving a dose rate of two megarads in 0.5 second. Again a hard film resulted.

EXAMPLE II A very viscous, resinous material, the condensation product of 2-hydroxyethyl acrylate, maleic anhydride and bis phenol a diglycidyl ether was coated on wood and passed under the 500 kilovolt electron beam as in Example I. The resinous oomposition was cured in 0.5 seconds to a hard and moderately tough film. The top surface of the film exposed to air was not tacky.

EXAMPLE III The resinous composition of Example II was dissolved in n-butyl acrylate in a ratio of 20 parts butyl acrylate to 80 parts resin. A piece of wood was coated with the resinous composition to a thickness of 0.005 inch and cured with a 300 kilovolt beam in 0.67 seconds to a hard and tough film. An 8-ounce steel ball dropped from a height of three feet onto the surface did not crack or craze the film. The top surface of the film exposed to air was slightly softer than the main film. Polishing this surface with carborundum dust removed this soft surface and left a hard, smooth surface with a reduction in film thickness of less than 0.001 inch.

Identical coatings cured in the same way but with a 5 mil Mylar film or 3 mil coated paper placed over the coated substrate had hard, cured surfaces.

EXAMPLE IV In a glass stoppered flask, 20 parts of polypropylene glycol was mixed with 14.2 parts of toluene diisocyanate. The flask became warm from a mild exothermic reaction. After standing for one hour, 12.4 parts of 2-hydroxyethyl acrylate were added. After standing overnight, a clear viscous liquid remained. A thin film of this resinous material was coated on aluminum, a 5 mil Mylar film placed over the coated substrate, and the sample passed under a 300 kilovolt electron beam. The coating cured in 0.33 second to a very flexible, tough film which adhered strongly to the aluminum.

A solution of 90% of the above reaction product in n-butyl acrylate gave a harder, very flexible film on curing under the 300 kilovolt electron beam for 0.67 seconds.

EXAMPLE V One hundred grams of Vorite 63 (a castor polyester diisocyanate prepolymer made by The Baker Castor Oil Company), 43 grams of Z-hydroxyethyl acrylate and 0.14 grams of tin octoate catalyst were mixed in a 250 ml. rotating flask at 50 C. for 2 /2 hours. Free isocyanate in the final solution analyzed 0.02%. A 0.010 inch thick film of the product cured in about 0.67 seconds by irradiation under a 300 kv. electron beam to a dose of 7 megarads. A hard, impact resistant coating resulted. A solution of 11.3 parts product and 3.5 parts n-butyl acrylate gave a liquid which cured as above to a flexible, impact resistant film.

EXAMPLE VI Five grams of DDI diisocyanate (an aliphatic long chain diisocyanate made by General Mills, Incorporated) was mixed with 1.93 grams of 2-hydroxyethyl acrylate and 0.04 grams of tin octoate at 90 C. for five minutes. Thin films of the product on aluminum and plywood were covered with Mylar film and irradiated to a dose of 7 megarads under a 300 kv. electron beam for about 0.67 seconds. The cured film was hard but very flexible.

Other resins, for example those described in U.S. Pat. 3,367,992 can also be cured in a short length of time with high energy electrons in the same manner as set forth in the above examples.

What is claimed is:

l. A method of coating a surface of a substrate with a flexible, impact resistant [vinyl ester] material comprising,

(a) spreading over the surface of the substrate a liquid [vinyl ester] coating composition containing a polymerizable resin having terminal vinyl ester groups made by reacting:

(l) a polyfunctional material selected from the group consisting of (a) dicarboxylic acids or acid chlorides having from 4 to 15 carbon atoms, and (b) [polyepoxides having terminal, reactive glycidyl groups, (c)] polyisocyanates having terminal, reactive isocyanate groups and (d) dicarboxylic acid esters of polyepoxides,] with Z-hydroxyalkyi acrylates or methacrylates; or

(2) a half ester of 2-hydroxyalkyl acrylates [s] or methacrylateIs] and a a'ibasic acid with a polyepoxide,- and (b) exposing for one second or less the liquid coating composition to a beam of electrons having energy and current density suflicient to impart to the coating composition and ionizing radiation dose of 1 to 10 megarads.

2. The method according to claim 1 wherein the coating composition includes 0 to 50% by weight of a vinyl monomer.

3. The method according to claim 2 wherein the coating composition includes 0 to 50% by weight of an acrylate monomer.

4. The method according to claim 3 wherein the acrylate monomer is an alkylacrylate having from 1 to 10 carbon atoms.

5. The method according to claim 3 wherein the acrylate monomer is diacetoneacrylamide.

6. The method according to claim 3 wherein the acrylate monomer is a cycloalkyl acrylate.

7. The method according to claim 1 wherein the liquid composition includes a pigment.

8. The method according to claim 1 wherein the liquid composition includes a dye.

9. The method according to claim 1 wherein the electron beam has an energy of at least 250,000 volts.

10. The method according to claim 1 wherein the [liquid coating composition contains] polymerizable resin is adipoyl-bis (2-oxyethylacrylate) and the coating composoin'on contains up to 40% by weight of acrylate monomer.

11. The method according to claim. 1 wherein the [coating composition contains] polymerizable resin is a half ester condensation product of Z-hydroxyethyl acrylate L] and maleic anhydride and] reacted with a bisphenol A polyepoxide, and the coating composition contains up to 40% by weight of acrylate monomer.

12. The method according to claim 1 wherein the [coating composition contains] polymerizable resin is the condensation product of a polyglycol, a polyfunctional isocyanate and 2-hydroxyethylacrylate, and the coating composition contains up to 40% by weight of acrylate monmer.

13. The method according to claim 1 wherein the substrate is wood.

14. A method as in claim 1 wherein the polyfunctional material comprises a dicarboxylic acid or acid chloride having from 4 to 15 carbon atoms.

[15. A method as in claim 1 wherein the polyfunctional material comprises a polyepoxide having terminal reactive glycidyl groups] 16. A method as in claim 1 wherein the polyfunctional material comprises a polyisocyanate having terminal, reactive isocyanate groups.

[17. A method as in claim 1 wherein the polyfunctional material comprises a dicarboxylic acid ester of a polyepoxide] 18. A method of coating a surface of a substrate with a flexible, impact resistant [vinyl ester] material comprising,

(a) spreading over the surface of the substrate a liquid vinyl ester coating composition comprising the condensation product of a half ester of Z-hydroxyalkyl acrylate 0r melhacrylate [J and a dibasic acid [anhydride, and] with a polyepoxide; and

(b) exposing for one second or less the liquid coating composition to a beam of electrons having energy and current density suflicient to impart to the coating composition an ionizing radiation dose of 1 to 10 megarads.

References Cited UNITED STATES PATENTS 2/1968 Bearden 260837 4/1969 Burlant 11793.31

ALFRED L. LEAVITI, Primary Examiner J. H. NEWSOME, Assistant Examiner US. Cl. X.R.

117-148, 161 K, 161 KP, 161 ZB; 204159.19; 260- 837 

